Lead substitute material for radiation protection purposes

ABSTRACT

The invention relates o a lead substitute material for radiation protection purposes. The lead substitute material comprises Sn, Bi and optionally W or compounds of said metals and the composition of the lead-substitute material is a function of the nominal lead equivalent value.

The invention relates to a lead substitute material for radiationprotection purposes in the energy range of an X-ray tube with a voltageof 60-125 kV.

Conventional radiation protection clothing for use in X-ray diagnosisusually contains lead or lead oxide as the protective material.

Substitution of other materials for this protective material isdesirable for the following reasons, in particular:

On the one hand, lead and its processing entail a significantenvironmental impact and owing to its very large weight, on the otherhand, lead necessarily entails a very large heavy weight of theprotective clothing and therefore a great physical burden on the user.

For years, therefore, attempts have been made to find a substitutematerial for lead in radiation protection. The use of chemical elementswith atomic numbers of from 50 to 76, or their compounds, haspredominantly been proposed for this.

DE 199 55 192 A1 describes a method for producing a radiation protectionmaterial from a polymer as the matrix material and the powder of a metalwith a high atomic number.

DE 201 00 267 U1 describes a highly elastic, lightweight, flexible,rubber-like radiation protection material, wherein additives of chemicalelements with an atomic number greater than or equal to 50, and theiroxides, are mixed with a special polymer.

To reduce the weight compared with conventional lead shields, EP 0 371699 A1 proposes a material which likewise contains elements with highatomic numbers, in addition to a polymer as the matrix. A large numberof metals are mentioned in this case.

Depending on the elements which are used, the attenuation factor or leadequivalent value (International Standard IEC 61331-1, Protective devicesagainst diagnostic medical X-radiation) of the material in questionshows a sometimes very pronounced dependency on the radiation energy,which is a function of the voltage of the X-ray tube.

The known radiation protection clothing made of lead-free materialtherefore has a more or less significant reduction in absorptioncompared with lead below 70 kV and above 110 kV. This means that, inorder to achieve the same screening effect as for a material containinglead, a higher weight per unit area of the protective clothing isnecessary for this range of the X-ray voltage.

The application range of commercially available lead-free protectiveclothing is therefore generally restricted.

In order to be able to substitute lead for radiation protectionpurposes, an absorption performance as close as possible to that of leadis necessary over a larger energy range, radiation protection materialsusually being categorised according to the lead equivalent value and theradiation protection calculations often being based on lead equivalentvalues.

It is an object of the invention to replace lead as a radiationprotection material with respect to its screening properties over anenergy range of an X-ray tube with a voltage of 60-125 kV, that is tosay over a larger energy range, and over a larger thickness range of thenominal lead equivalent values, while simultaneously achieving a weightreduction which is as great as possible. Only materials which are moreenvironmentally friendly than lead are intended to be used in this case.

The object of the invention is achieved by a lead substitute materialfor radiation protection purposes in the energy range of an X-ray tubewith a voltage of 60-125 kV, which is characterised in that the leadsubstitute material comprises Sn, Bi and optionally W, or compounds ofthese metals, and the composition of the lead substitute material is afunction of the nominal lead equivalent value.

In order to achieve the object it was therefore necessary, on the onehand, to find a material selection for optimum screening properties overa larger energy range and, on the other hand, to find a materialselection for a larger thickness range of the protective layer.

Preferred compounds of Sn, Bi and W are their oxides.

It is a fundamentally new and surprising discovery that, in order toachieve an optimum result, the composition of lead substitute materialsvaries as a function of the thickness of the protective material. Alead-free screening material with the extended application range can nowbe achieved by a combination of tin with bismuth and optionallytungsten, which is matched to the respective nominal lead equivalentvalue.

In a preferred embodiment of the invention, the lead substitute materialis characterised in that it has 10-20% by weight of a matrix material,50-75% by weight of Sn, or Sn compounds, and 20-35% by weight of Bi, orBi compounds, for nominal lead equivalent values of up to 0.15 mm, and40-60% by weight of Sn, or Sn compounds, 15-30% by weight of Bi, or Bicompounds and 0-30% by weight of W, or W compounds, for nominal leadequivalent values of 0.15-0.60 mm.

In a particularly preferred embodiment of the invention, the leadsubstitute material is characterised in that it has 52-70% by weight ofSn, or Sn compounds, and 21-32% by weight of Bi, or Bi compounds, fornominal lead equivalent values of up to 0.15 mm, and 42-57% by weight ofSn, or Sn compounds, 15-30% by weight of Bi, or Bi compounds, and 5-27%by weight of W, or W compounds, for nominal lead equivalent values of0.15-0.60 mm.

The matched combination of tin and bismuth and optionally tungsten, orcompounds of these metals, now makes it possible to provide anenvironmentally friendly lead substitute material which is substantiallymore lightweight than conventional lead or lead oxide material, andwhich can substitute for the latter in the energy range of an X-ray tubewith a voltage of 60-125 kV. This energy range is the essential rangefor X-ray diagnosis.

The criterion when substituting for lead is a 10% deviation of the leadequivalent value from the nominal value, as stipulated in DIN 6813.Radiation protection clothing which is made of the substitute materialaccording to the invention can therefore be worn without restrictions inall applications of X-ray diagnosis. This constitutes a substantialadvantage over all known lead substitute materials.

In another particularly preferred embodiment of the invention, the leadsubstitute material is characterised in that it comprises a structuremade up of layers with differing composition.

The lead substitute material may comprise a structure made up of atleast two layers with differing composition, which are separate orconnected together, the layer further away from the body comprisingpredominantly Sn and the layer(s) near the body comprising predominantlyBi and optionally W.

The invention will be explained in more detail with reference to thefollowing examples and comparative examples.

The measurements of the weight- and energy-related radiation protectioneffects were based on the IEC 61331-1 standards; particular points tonote in this regard are the measurement geometry and the prefilteringmentioned therein for the X-radiation.

The results of the measurements are collated in Table 1 and in FIG. 1.TABLE 1 Weight per unit area (kg/m²) of the various radiation protectionmaterials, expressed in terms of the absorption by pure lead, undermeasurement conditions according to IEC 61331-1 as a function of energy.Protective material 60 kV 80 kV 100 kV 125 kV 150 kV Absorption of the97.2 89.3 80.8 74.4 69.7 primary radiation in % 0.25 mm of pure 2.832.83 2.83 2.83 2.83 lead (without matrix) - reference value Lead withmatrix 3.59 3.59 3.59 3.59 3.59 Commercially 3.46 2.88 2.96 3.63 4.41available lead-free material (Optimit R-100A) Commercially 3.79 3.093.20 4.13 4.51 available lead-free material (Xenolite ® NL) Leadsubstitute 2.93 2.83 2.83 3.07 3.53 material according to the invention,with the composition: 15 wt. % matrix, 54 wt. % Sn, 12 wt. % W, 19 wt. %Bi

Table 1 shows that, for an equal protective effect in the range of60-125 kV, the lead substitute material according to the invention hasthe most advantageous weight per unit area of all the lead-freematerials.

A radiation protection shield with the nominal lead equivalent value0.25 mm, made of the novel material, therefore weighs around 21% lessthan a conventional shield with lead as the protective material.

FIG. 1 shows the relative weight per unit area of the various protectivematerials in Table 1, expressed in terms of the absorption by pure leadin the energy range 50-150 kV.

FIG. 2 shows the application range determination of the lead substitutematerial according to the invention in Table 1, based on a 10% deviationof the lead equivalent value at 80 kV. The determination is carried outaccording to DIN 6813 and gives an application range of at least 60-125kV for the material specified.

The measurements which were carried out furthermore show that theradiation-physical properties of the lead substitute material aredependent both on the energy of the incident radiation and on the layerthickness, that is to say the composition of the lead substitutematerial needs to be modified for each layer thickness, in order tomatch it to the absorption performance of lead.

The results are shown in Table 2, where the compositions are indicatedfor conventional lead equivalent values with the corresponding valuesmeasured according to IEC 61331-1. Weight Nominal lead Composition perunit 60 kV 80 kV 100 kV 125 kV equivalent value M = Matrix area Beamqualities according to IEC 61331-1 (mm) material (kg/m²) Measured leadequivalent value (mm) 0.025 65 wt. % Sn + 22 wt. % 0.25 0.023 0.0250.025 0.023 Bi + 15 wt. % M 0.05 55 wt. % Sn + 30 wt. % 0.51 0.045 0.0500.050 0.045 Bi + 15 wt. % M 0.125 55 wt. % Sn + 30 wt. % 1.25 0.1200.125 0.125 0.120 Bi + 15 wt. % M 0.25 54 wt. % Sn + 12 wt. % 2.8 0.240.25 0.25 0.23 W + 19 wt. % Bi + 15 wt. % M 0.35 48 wt. % Sn + 20 wt. %3.9 0.33 0.35 0.36 0.32 W + 17 wt. % Bi + 15 wt. % M 0.05 44 wt. % Sn +25 wt. % 5.5 0.48 0.50 0.50 0.45 W + 16 wt. % Bi + 15 wt. % M

As can be seen from Table 2, for example, the substitute materialcomparable with 0.2 mm of lead consists of 15% by weight matrixmaterial, 54% by weight Sn, 12% by weight W and 19% by weight Bi, with amass per unit area of 2.8 kg/m² in total. The matrix material is thesubstrate and may, for example, consist of rubber or latex. Largedeviations from the composition according to the invention detrimentallyeffect either the allowable application range and/or the weight.

If a protective layer with a lead equivalent value of 0.5 mm isrequired, however, then the composition needs to be modified accordingto Table 2 in order to achieve the performance corresponding to leadover an energy range of from 60 to 125 kV.

In terms of radiation physics, the embodiment of the invention to whichclaim 5 relates can make it possible to further reduce the user'sradiation exposure. For example, the radiation exposure at an X-rayvoltage of 100 kV can be reduced by about 15% if the outer layerconsists exclusively of tin and the inner layer consists of bismuth andoptionally tungsten. The weight of the protective clothing canadvantageously be reduced further by taking this relationship intoaccount.

1. Lead substitute material for radiation protection purposes in theenergy range of an X-ray tube with a voltage of 60-125 kV, characterisedin that the lead substitute material comprises Sn, Bi and optionally W,or compounds of these metals, and the composition of the lead substitutematerial is a function of the nominal lead equivalent value.
 2. Leadsubstitute material according to claim 1, characterised in that itcomprises 10-20% by weight of a matrix material, 50-75% by weight of Sn,or Sn compounds, and 20-35% by weight of Bi, or Bi compounds, fornominal lead equivalent values of up to 0.15 mm, and 40-60% by weight ofSn, or Sn compounds, 15-30% by weight of Bi, or Bi compounds, and 0-30%by weight of W, or W, compounds for nominal lead equivalent values of0.15-0.60 mm.
 3. Lead substitute material according to claim 2,characterised in that it comprises 10-20% by weight of a matrixmaterial, 52-70% by weight of Sn, or Sn compounds, and 21-32% by weightof Bi, or Bi compounds, for nominal lead equivalent values of up to 0.15mm, and 42-57% by weight of Sn, or Sn compounds, 15-30% by weight of Bi,or Bi compounds, and 5-27% by weight of W, or W compounds, for nominallead equivalent values of 0.15-0.60 mm.
 4. Lead substitute materialaccording to any one of the preceding claims, characterised in that itcomprises a structure made up of layers with differing composition. 5.Lead substitute material according to claim 4, characterised in that itcomprises a structure made up of at least two layers with differingcomposition, which are separate or connected together, the layer furtheraway from the body comprising predominantly Sn and the layer(s) near thebody comprising predominantly Bi and optionally W.